Feature: Make context available in server function validators

[dxptqhtlutehvlyxcmtg]

It'd be great if server function validators could receive the current request context. Currently, it seems impossible to have "runtime" schemas that depend on the current user's information, say currency type or time zone.

Unlike handlers, which can read the session supplied by middleware via the context, I don't see a way for validators to access that context.